In the recent past, several types of superconducting rotary machines—either as generators or as motors—have been proposed or even built. Their potential application lies in the field of wind power, other power generation, such as hydro power, as well as motors for machinery, ship propulsion, trains or large road vessels.
In the field of wind power, the size and capacity of wind turbines have increased over the recent past in an attempt to reduce the cost of wind energy and to reduce the top head mass, especially for floating foundations. As a result thereof, there is a need for reducing the size and weight of such wind turbine generators. In recent developments, superconductive generators have been proposed for use in wind turbines since they are smaller and lighter and have a higher power density than conventional copper based generators. Lately, superconductors with elevated critical temperatures have been proposed for use in wind turbines. Such critical temperatures, Tc, define the maximum temperature where the superconducting effect occurs. Exemplary materials, without limiting to them, are MgB2 (Tc around 40 K) or YBCO (Tc around 95 K). Particularly, the latter are referred to as high temperature superconductors (HTS). Operating temperatures for all these materials are in the range of 15 K to 35 K, or even above, and make them suitable for use in generators since they can be cooled by using simple cooling machines.
Conventional copper based synchronous generators comprise a rotor having a plurality of projecting poles (called salient poles) with copper coils wrapped around the side surfaces of each core element. The surrounding coils fill up the space between adjacent poles so that the arrangement of poles forms a more or less flushed outer surface. The poles are commonly formed as an integrated part of the back iron. The poles or the top end thereof can also be mounted directly to the back iron by use of fastening means such as screws or bolts. Since the copper windings are operated at temperatures much higher than the cryogenic temperatures at which the superconductive windings are operated, there is no need for thermally separating the poles from the rest of the rotor assembly. An expert in the art will avoid any distance between such a pole element and the back iron because it weakens the magnetic circuit due to increasing the air gap.
Various superconductive synchronous generators are described in the literature, such as in EP 1305871 B1, which discloses a rotor with superconductive windings wrapped around a stepped pole element in direct thermal connection with the back iron. In this configuration, the poles and the back iron form a large thermal mass that needs to be cooled to operating temperatures by the cooling system which in turn increases the cool down time.
EP 2521252 A1 discloses a rotor with integrated projecting pole elements around which superconductive coils are wrapped. Each set of coils is enclosed in a vacuum chamber of a ladder-shaped cryostat. This cryostat allows the superconducting coils to be kept cold while the poles and back iron are at ambient temperature. The projecting poles are arranged in main openings of the cryostat where an electromagnetic shield is provided on the outer surfaces of the pole elements and cryostat. This solution provides a complex configuration that increases the total assembly time and costs.
US 2008/0079323 A1 discloses an electrical machine comprising a rotor provided with superconductive coils. These superconductive coils are held in place by a support bracket where this support bracket is spaced apart from the back iron by means of a plurality of thermally insulating blocks located on opposite sides of the bracket. The support bracket is further connected to the back iron by means of thermally insulating bolts extending radially inwards towards the back iron. The support bracket and bolts are slidably arranged in axial extending notches located in the blocks and in the back iron respectively. However, the nuts of said bolts along with the additional bolts used to fixate the blocks are only accessible from a radial direction.
EP 2731232 A1 discloses a permanent magnet (PM) generator wherein the pole units of both the stator and the rotor are arranged in a removable active unit. Matching coupling elements in the form of grooves and projecting dovetails are used to axially slide to the respective pole units into position relative to the stator and the rotor. As this PM-generator is operated at much higher temperatures than a superconductive generator, no thermally insulating structure is needed.
The cited documents all disclose a rotor assembly that is assembled from a radial direction of the rotor. This further provides a complex and time consuming repair process as the rotor needs to be removed from the stator in the event of a repair. Therefore, there exists a need for a simpler and easier method of manufacturing and repairing such superconducting rotary machines.